IMPALA-1589: interpreted path for vararg functions in ScalarFunctionCall

Allows vararg functions to be used without codegen. Note that this
requires the fix for IMPALA-1596.

Change-Id: Ibf3de04728da91db04bbd9ec5ea02623bea1b41e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5548
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: Skye Wanderman-Milne <skye@cloudera.com>